The present polarization mode dispersion compensator is based on delay generators that use thermally actuated, rotating micro-mirrors to add well-defined increments of delay to polarized portions of an optical signal.
Fiber optics technology is revolutionizing the telecommunications field. The main driving force is the promise of extremely high communications bandwidth. At high bandwidths, a single beam of modulated laser light can carry vast amounts of information equal to hundreds of thousands of phone calls or hundreds of video channels. However, pulse broadening limits the effective bandwidth and propagation distance of an optical communication signal. Because of the inherent dispersive nature of an optical fiber medium, all portions of a light pulse do not travel the same distance through an optical fiber causing pulse broadening.
FIG. 1 illustrates how pulse broadening arises from varying light propagation delays, which eventually distorts light output. Digital input pulses 10 are input to an optical fiber medium 11. The amplitude-modulated pulses are generated by a modulated laser source, such as a direct-modulated laser or an externally-modulated laser.
Different portions of a light pulse encounter varying propagation delays arising from the varying lengths of reflected paths within optical fiber 11. For clarity, three paths are illustrated which correspond to a relatively straight, short path 10a, a reflected, intermediate length path 10b, and a relatively long, reflected path 10c. Due to the varying propagation delays (see, e.g., the xcex94t delay in arrival time between 10a and 10b), the combined optical output is distorted. Thus, a photoreceptor detecting the output pulses 10a-10c will generate a distorted output 12.
As shown in FIG. 2, such pulse broadening can lead to intersymbol confusion. xe2x80x9cPulse broadeningxe2x80x9d is called xe2x80x9cdispersionxe2x80x9d or xe2x80x9cspreadingxe2x80x9d because of the non-uniform way in which parts of the incident signal 20 propagate through a dispersive fiber medium. In a mild form of dispersion, the transitions between ON and OFF states observed at a receiver are not as abrupt and distinct as the transitions that were originated by a transmitting laser. More severe blurring in the time domain limits the useful bandwidth of the path.
In FIG. 2, dispersion effects have broadened two closely spaced pulses to the extent that they are almost indistinguishable, as indicated by a question mark in the output signal 22. This will cause an information bit to be received erroneously, with perhaps disastrous results on network communication and customer dissatisfaction.
Several refinements have been made to reduce dispersion and increase the useful bandwidth. First, single-mode fiber was developed having a slender core such that there is essentially only a single light path through the fiber. Secondly, the distributed feedback (DFB) laser was developed with an extremely narrow distribution of output wavelengths. This technique minimizes chromatic dispersion caused by the fact that different wavelengths traverse the length of the fiber over different periods of time. Finally, a dispersion-shifted fiber material was produced to minimize the increased time vs. wavelength dependency at a specific wavelength of fifteen hundred and fifty nanometers common in telecommunication applications.
Cumulatively, recent improvements in fiber materials and transmitter devices have reduced pulse dispersion and increased working bandwidth. Lightwave technology has advanced at such a pace that the bandwidth capabilities have more than doubled every two years. As a result, working bandwidths, expressed in terms of digital bit-per-second rates, have escalated from 500 Million bits per second (Mbps) to 10 Billion bits per second (Gbps).
These progressively more exotic refinements have brought the technology to a new bandwidth barrier: Polarization-Mode Dispersion (PMD). Previously, PMD was insignificant in magnitude relative to other dispersive effects, but now it is a limiting factor. It is well known that light can be polarized and that, for a given beam of light, this polarization can be expressed in terms of two orthogonal axes that are normal to the axis of propagation. As a beam of light propagates through a fiber, the light energy present along one such polarization may leak into the other polarization.
This leakage would normally be of little consequence (lightwave receivers will detect both polarizations), except that real-world fibers carry different polarizations at slightly different time delays due to reflection. This effect can be on the order of 10-20 picoseconds (ps) in a 100 km fiber and becomes important when the modulating pulses are 50-100 picoseconds in width. To complicate matters, the polarization dispersion within a given fiber changes as a function of time and temperature. Therefore, an effective PMD compensation mechanism must monitor and adapt to the changes so as to keep PMD to a minimum.
To nullify the effects of PMD, researchers have suggested application of an adaptive compensation device in an optical path at the receiving end just before the receiving transducer. These compensators typically employ a detector for analyzing the relative partitioning and delay of the incoming signal along two orthogonal polarizations. The compensators correct a data signal by purposefully adding delay selectively to one polarization or another. A controller interprets the findings of the delay analyzer and manipulates adjustable delay elements so as to compensate for the polarization-dependent delay differences caused by the imperfect fiber transmission path. However, these techniques are not practical in telecommunication applications, such as, long-haul optical fiber communication.
The variable delay elements are usually optical fibers that are either heated or squeezed to alter their propagation characteristics. While these elements are adaptable to laboratory electronic control techniques, they are inadequate in terms of reproducibility and predictability of response. They are also impractical for use in a commercial traffic-bearing fiber network wherein recovery time following an equipment or power failure should be minimized. (See, e.g., Ozeki, et al., xe2x80x9cPolarization-mode-dispersion equalization experiment using a variable equalizing optical circuit controlled by a pulse-waveform-comparison algorithm,xe2x80x9d OFC""94 Technical Digest, paper TuN4, pp. 62-64; Ono, et al., xe2x80x9cPolarization Control Method for Suppressing Polarization Mode Dispersion Influence in Optical Transmission Systemsxe2x80x9d, Journal of Lightwave Technology, Vol. 12, No. 5, May 1994, pp. 89-91; Takahasi, et al., xe2x80x9cAutomatic Compensation Technique for Timewise Fluctuating Polarization Mode Dispersion in In-line Amplifier Systemsxe2x80x9d, Electronics Letters, Vol. 30, No. 4, February 1994, pp. 348-49; and WO 93/09454, Rockwell, Marshall A.; Liquid Crystal Optical Waveguide Display System).
U.S. Pat. No. 5,859,939 (Fee et al.) discloses a polarization beam splitter that separates the optical data signal into first and second orthogonally polarized optical signals. A first variable time delay element provides a first incremental propagation delay for the first polarized optical signal. A second variable time delay element provides a second incremental propagation delay for the second polarized optical signal. The first and second variable time delay elements consist of a series of optical switches optically interconnected by different incremental lengths of optical fiber. For example, 2xc3x972 optical switches are provided for switching between a reference fiber segment and a respective delay fiber segment to provide a relative incremental propagation delay. A controller controls optical switches in the first and second variable switching delay elements to set first and second incremental propagation delays. The transition to and from the optical switches is a source of signal loss.
What is needed is a PMD compensation method and system that is reliable, responsive, and effective in commercial telecommunication networks.
The present invention provides a system and method for compensating for polarization mode dispersion (PMD) in an optical data signal using rotating micro-mirrors to provide incremental delays between different polarization modes of the optical data signal. The PMD compensator receives a signal and breaks the signal into its various polarization modes. A variable delay generator provides an appropriate time delay to one or more of the polarization modes. A controller monitors the polarization mode dispersion and positions rotating micro-mirrors to provide the required time delay.
In one embodiment, a polarization mode separator separates the optical data signal into first and second orthogonally polarized optical signals. Rotating micro-mirrors in a variable delay generator are positioned to direct the first orthogonally polarized optical signal to optical paths of various lengths. The longer the optical path, the longer the first propagation delay for the first polarized optical signal. In another embodiment, a variable delay generator is provided for each of the first and second orthogonally polarized optical signals. Consequently, delay can be introduced into the first and/or the second polarized optical signal.
In particular, the first and second polarized optical signals are incrementally delayed relative to one another so as to compensate for polarization mode dispersion. A beam combiner then combines the first and second polarized optical signals to form an optical output data signal that can be detected accurately and reliably by a receiver without the effects of polarization mode dispersion. In this way, optical data signals can be transmitted over greater distances along a long-haul fiber optic dispersive medium at even greater bit-rates and bandwidth.
In one aspect of the present invention, a beam splitter diverts a portion of an input optical data signal to a delay detector. The delay detector detects a relative delay between orthogonal polarization modes of the optical data signal due to polarization mode dispersion. The controller then uses the detected relative delay to control the rotating micro-mirrors in the variable delay generators so as to counteract the detected relative delay.
In one embodiment, a linkage mechanism is mechanically coupled to the two rotating micro-mirrors. A plurality of thermal actuators are mechanically coupled to the linkage mechanism. The linkage mechanism can rotate the micro-mirrors simultaneously in opposite directions. The linkage mechanism can also synchronize rotation of the micro-mirrors.
The present invention is also directed to a plurality of variable delay generators on the substrate and to an optical communication system including at least one apparatus for equalizing polarization mode dispersion.
Compared to other known technologies, the present invention is more reliable and predictable in its response and is therefore more mass-producible. Furthermore, it has an extremely fast response time that is independent of the degree of delay adjustment needed. This is a particular advantage in a mission-critical high data rate optical communications network.